EPDs (Electrophoretic Displays) and their use for electronic paper are known for a number of years. An EPD generally comprises charged electrophoretic particles dispersed between two substrates, each comprising one or more electrodes. The space between the electrodes is filled with a dispersion medium which is a different colour from the colour of the particles. The dispersion medium is usually a low refractive index solvent, such as dodecane. Fluorinated solvents may be used for example in Total Internal Reflection (TIR) type EPDs. If a voltage is applied between the electrodes, charged particles move to the electrode of opposite polarity. The particles can cover the observer's side electrode, so that a colour identical to the colour of the particles is displayed when an image is observed from the observer's side. Any image can be observed using a multiplicity of pixels. Mainly black and white particles are used. Available technologies of EPDs include electronic paper, commercially used in electronic books. This application uses black and white colour.
The use of different coloured particles in a single pixel has been exemplified in recent patent literature (U.S. Pat. No. 7,304,634, GB 2 438 436, US 2007/0268244). Two particle systems comprising inorganic and resin particles are also known (EP 1 491 941). These coloured particles are only achievable by complicated processes and/or they are only suitable for specific applications. Particles comprising a polymer and a pigment or a dye prepared by an evaporative process are described in US 2010/120948, WO 2011/154103, WO 2011/154104, WO 2013/026519, Nippon Gazo Gakkaishi 46(4) 2007, 247-253, and Kobunshi Ronbunshu, 62(7), 310-315 (July 2005).
However, there still is a need for a simple, repeatable and cheap preparation of fluids comprising black polymer particles dispersed in low refractive index media, especially in a fluorinated media, wherein the black polymer particles do not leach colour in a dispersion and preferably show electrophoretic mobility. An improved route to provide black particles and new fluids comprising such particles has now been found.